This invention relates to a motorcycle and, more particularly, to an inverted type front fork structure for a motorcycle having an improved structure for achieving a stable running performance.
An inverted type front fork assembly connected to a steering shaft of a motorcycle through a bracket generally comprises two fork members disposed on bilateral sides of the motorcycle, each front fork comprising an outer tube connected to the steering shaft through a bracket means, generally including an upper bracket and an under bracket, an inner tube shrinkably inserted into the inner hollow portion of the outer tube from the lower side thereof, an axle bracket attached to the lower end of the inner tube, a front axle penetrating the axle bracket in a supported manner to which a front wheel and a disc plate are operatively connected, and a caliper connected to the axle bracket through a caliper bracket.
As stated above, the front fork assembly comprises two fork members each serving as a shock absorbing member, and each front fork half is referred to as a front fork or front fork assembly hereunder throughout the present specification for the sake of convenience.
A flexible brake hose for transmitting hydraulic pressure to the caliper from a brake master cylinder is arranged so as to extend on one side along the longitudinal side of the outer and inner tubes and extend below the lower end of the inner tube. The lower end portion of the brake hose extends along the lower side of the caliper and is connected thereto and, accordingly, there may is a likelihood of the brake hose colliding with a road surface or an obstacle on the road during the running of the motorcycle in spite of the fact that the bent angle of the flexible brake hose is made relatively large at the downward end of the front fork protector. In order to obviate this problem, there is also provided a brake hose arrangement in which the lower end portion of the brake hose further extends from the lower portion of the inner tube upwardly towards the connecting portion of the caliper along the upper configuration thereof for preventing this portion of the brake hose from contacting a road surface during the running of the motorcycle. In this case where the lower end portion of the flexible brake hose extends along the upper configuration of the caliper, contact of the lower end portion of the brake hose with the road surface may be prevented. However, when running direction of the front wheel is changed, running direction, the outer tube is shrunk downwardly relative to the inner tube. The brake hose supported to the outer tube is also lowered. The arrangement of the flexible brake hose is made complicated in a bent manner, for example, in an S-shape, for the reason described above to prevent the same from contacting the road surface when lowered. The bent portion of the flexible brake hose is disposed inside the axle bracket, i.e. on the side of the disc plate, to prevent the bent portion from protruding outwardly.
The downwardly extending portion of the flexible brake hose along the front fork is disposed behind a front fork protector of substantially a flat plate structure arranged in front of the inner tube.
According to the front fork braking mechanism of the character described above, the downward end of the flexible brake hose extending behind and along the front fork protector is bent rearwardly at the lower end portion of the inner tube at severe angles with a small bending radius because the front fork protector has a flat plate-shape. This presents problems such that the entire length of the flexible hose is unnecessarily elongated, resulting in the swelling of the brake hose due to the inner hydraulic pressure when the front wheel is braked and, in addition, the pressure loss of the braking pressure may be accordingly increased.
With the inverted type front fork of the construction described, it is necessary to stably and safely maintain the damper effect in the outer tube due to the sliding motion of the inner tube inside the outer tube, and for this purpose, the clamping force of the under bracket to the lower portion of the outer tube is usually restricted to a force below a predetermined value, whereas the clamping force of the upper bracket to the upper portion of the outer tube is free from the magnitude of the clamping force because the upper portion of the outer tube spaced is apart from the sliding portion of the inner tube inside the outer tube. However, it is troublesome to make different clamping forces for the upper and under brackets during assembly and such complicates the assembly process.
In other words, in a case where the clamping force of the upper bracket is made substantially equal to that of the under bracket, the outer tube may be upwardly displaced when the impact from the road surface is caused during the running of the motorcycle. A normal impact can be absorbed by the damper effect due to the upward displacement of the under bracket, but when a further impact is applied after the under bracket is displaced to its uppermost position, the impact is transferred directly to the outer tube and, accordingly, in a case where the clamping force of the upper bracket to the outer tube is below the predetermined value, the outer tube may be displaced upwardly together with the front wheel. The under bracket is in turn secured to the upper surface of a front fender disposed above the front wheel. In a case where the front wheel is upwardly to a larger extent displaced, the front wheel may contact or abut against the inner surface of the front fender, which may cause a locking condition of the front wheel during the running of the motorcycle. This results in an extremely dangerous condition.
The front fork protector is arranged for the purpose of protecting the inner tube so as to maintain a good damper effect of the inner tube. The front fork protector is secured at the lower end to the lower portion of the inner tube through a bracket and supported at the upper end to the outer tube through a protector guide in the form of a frame to be vertically slidable but restricted in the to-and-fro direction and the bilateral direction thereof. Namely, the protector guide is supported to the outer tube to be relatively vertically movable together with the inner tube with respect to the outer tube.
With the conventional front fork protector including the protector guide of the type described above, the protector guide is rotated in a case where the motorcycle is turned over during the running or contacts another motorcycle or the like. The rotation of the protector guide results in a large the friction loss due to contact with the front fork protector and, in an adverse case, the front fork protector may be damaged or broken. When the motorcycle is assembled, it is necessary to substantially align the center of the front fork protector with the center of the protector guide to minimize the friction loss, but the positioning of the protector guide for this purpose is troublesome and increases the assembly process.